Normally, hybrid circuits are comprised within data communications equipment (DCE) located at a central office. A four-wire circuit contained in the DCE connects the hybrid circuit to the receiver and transmitter of the DCE. A two-wire circuit, which corresponds to the subscriber loop, couples customer premises to the hybrid circuit at the central office. The hybrid circuit performs a conversion from the two-wire circuit to the four-wire circuit, and vice versa. The hybrid circuit provides the capability of full-duplex communication between the central office and the customer premises.
A typical hybrid circuit comprises a bridge circuit that is coupled to the two-wire circuit via a transformer. The bridge circuit comprises an impedance matching network for matching the impedance of the hybrid circuit with the impedance of the subscriber loop. Transhybrid loss, which corresponds to the portion of the signal transmitted by the transmitter of the DCE that is sent over the subscriber loop without being received by the receiver of the DCE, is controlled by matching the impedance of the impedance matching network of the bridge circuit with the impedance of the subscriber loop. Therefore, it is desirable to maximize the transhybrid loss, thus minimizing the amount of the transmitted signal which is reflected at the interface of the bridge circuit and the subscriber loop and received by the receiver of the DCE, which is commonly referred to as near-end echo.
In the past, attempts have been made to maximize the transhybrid loss by designing the impedance of the impedance matching network of the hybrid circuit so that it matches, as closely as possible, the impedance of the subscriber loop. However, once the impedance matching network was designed and implemented in the hybrid circuit, the impedance of the impedance matching network was fixed and could not be altered.
Accordingly, a need exists for a hybrid circuit having a variable impedance matching network that can be implemented in a practical manner and that is cost-effective.